1. Field of the Invention
The present invention relates generally to offset cancellation in amplifiers, and more specifically to method and device for performing offset cancellation in an amplifier using floating-gate transistors.
2. Description of Related Art
An operational amplifier, usually referred to as an ‘op-amp’, is a DC-coupled high-gain electronic voltage amplifier with differential inputs and, usually, a single output. In its ordinary usage, the output of the op-amp is controlled by negative feedback which, because of the amplifier's high gain, almost completely determines the output voltage for any given input. Op-amps are among the most widely used electronic devices today, being utilized in a vast array of consumer, industrial and scientific devices. General-purpose integrated op-amps of standard specification sell for well under one U.S. dollar. Modern designs are electronically more rugged than earlier implementations and some can sustain direct short-circuits on their outputs without damage.
A practical concern for op-amp performance is voltage offset. That is, effect of having the output voltage something other than zero volts when the two input terminals are shorted together. Operational amplifiers are differential amplifiers which are designed to amplify the difference in voltage between the two input connections and nothing more. In an ideal situation, when that input voltage difference is exactly zero volts, zero volts are expected to be present on the output. However, in the real world the ideal case rarely happens, even if the op-amp in question has zero common-mode gain, the output voltage may not be at zero when both inputs are shorted together.
Mismatches between MOS transistors pose a serious challenge to analog circuit designers and most commonly manifest themselves as an offset voltage in operational amplifiers. Techniques commonly used to reduce the offset voltage include auto-zeroing, correlated double sampling and chopper stabilization. Auto-zeroing and correlated double sampling are techniques applicable to sampled data systems while chopper stabilization allows continuous-time operation of the amplifier. Alternate techniques include resistor trimming through the use of poly-fuses or laser trimming. These typically involve special processing steps and are usually expensive.